1. Field of the Invention
The present invention relates to a communication system in which the amount (number of bits or length) of information stored in a transmission frame can dynamically vary for each transmission frame.
2. Description of the Related Art
Recently, mobile communications have rapidly spread and a variety of communication methods have been proposed and developed. One next-generation cellular phone system (IMT-2000), W-CDMA (Wideband-Code Division Multiplex Access) is proposed.
In W-CDMA, a variety of new technologies are introduced to facilitate a high transferrate, increasing number of users, effective use of the radio band. A technology called “BTFD (Blind Transport Format Detection)” is one such newly-adopted technology.
A system adopting BTFD can dynamically change the data rate while transmitting information. Specifically, while information is stored in a frame and transmitted, the number of information bits (that is to say, information length) in a frame can be changed for each frame. In this case, a transmitter does not notify a receiver of the information length of a frame. Therefore, when obtaining information from a received frame, the receiver must detect the information length of the frame.
FIG. 1 shows an example format of a transmission frame used in BTFD. A CRC (Cyclic Redundancy Code) is appended to each piece of information to be transmitted (information bits), and error correction encoding is applied to the information. The error correction code is, for example, a convolutional code. Then, the encoded data is stored in a transmission frame and is transmitted over a network. In the following description, a transmission frame transmitted over a radio network is sometimes called a “radio frame”. Basically, the length of a radio frame is fixed. Therefore, when encoded data is stored in a radio frame, DTX (Discontinuous Transmission) is set, if necessary, in order to meet the space requirements of the data area of the radio frame. “DTX” means not to transmit a signal.
In this way, in BTFD, the information length of each radio frame can be changed for each frame. Then, the receiver detects the information length on receipt of the transmitted information.
FIG. 2 is a flowchart showing the general algorithm of BTFD, which is executed by a receiver. It is assumed here that the system uses the three information lengths shown in FIG. 1 (50 bits, 100 bits and 200 bits).
On receiving a signal (radio frame) in step S1, the process for detecting the information length of the radio frame is started. In steps S2 and S3, error detection is performed assuming that the information length of the received radio frame is 50 bits. This error detection is, for example, a CRC operation. In step S3, if no error is detected, it is judged that the information length of the received radio frame is 50 bits, and the 50 bits of information are obtained. If an error is detected in step S3, the flow proceeds to step S4.
In steps S4 and S5, error detection is performed assuming that the information length of the received radio frame is 100 bits. In step S5, if no error is detected, it is judged that the information length of the received radio frame is 100 bits, and the 100 bits of information are obtained. If an error is detected in step S5, the flow proceeds to step S6.
In steps S6 and S7, error detection is performed assuming that the information length of the received radio frame is 200 bits. In step S7, if no error is detected, it is judged that the information length of the received radio frame is 200 bits, and the 200 bits of information are obtained. However, if an error is detected in step S7, it is judged that an error in the transmission line occurs, and a process corresponding to the error is performed in step S8.
In this way, in a communication system adopting BTFD, a receiver detects the information length of a radio frame every time a frame is received and it obtains the information accordingly. As described above, in existing BTFD, when an error detection process with a prediction of an information length of received frame is repeated, the order of the predicted information length used in the error detection process is fixed.
For a detailed description of BTFD, see for example 3rd Generation Partnership Project; Technical Specification Group Radio Access Network, Multiplexing and channel coding (3G TS 25.212 version 3.1.0).
If the algorithm shown in FIG. 2 is adopted, an error detection process is performed only once when 50 bits of information are stored and transmitted in a radio frame. However, if 100 bits of information are stored and transmitted in a radio frame, the error detection process is performed twice, and if 200 bits of information are stored and transmitted in a radio frame, the error detection process is performed three times. Therefore, in this case, the average number of times that the error detection process will be performed is twice per radio frame.
However, before performing this error detection, generally, error correction decoding, in which the operation amount is large, must be performed. Therefore, if the number of times that the error detection process is performed increases, it requires a longer processing time and a larger amount of power. As a result, it is advantageous to minimize the number of times that the error detection process is performed.
Although in the above example, it is assumed that the system uses three kinds of information lengths, it follows that the average number of executions of the error detection process will increase, as the number of information lengths increase.